1. Field of the Invention
The present invention relates to a method for forming an insulating film on the surface of a semiconductor substrate and to an apparatus for carrying out the method.
2. Description of the Related Art
Conventionally, silicon oxynitride films are used as gate insulation films and capacitor insulating films for semiconductor devices, particularly, when they are silicon devices, MOS (Metal Oxide Semiconductor) transistors and MOS capacitors. These insulating films must have a high dielectric breakdown voltage and a high dielectric breakdown charge amount. A wafer cleaning process plays an important role in attainment of the requirement, as wafers must be properly cleaned and have a low fixed electric charge density and a low interface state density.
Along with a recent tendency to reduce the geometry and increase integration of semiconductor device circuits, gate insulating films and capacitor insulating films are becoming thinner. For example, under the design rule of 0.1 .mu.m or less, gate insulating films must be as thin as 3 nm or less.
According to a conventional method for forming gate insulating films of MOS transistors, a semiconductor substrate is exposed to an atmosphere of dinitrogen monoxide (N.sub.2 O) or nitrogen monoxide (NO) at a high temperature of about 1000.degree. C. Alternatively, a wafer is heated to a temperature of about 700.degree. C. in an ammonia atmosphere.
Also, conventional methods for forming oxynitride films at low temperatures include the following: thermal oxynitridation is performed while ultraviolet rays are radiated; and silicon is directly nitrided through exposure to nitrogen compound plasma or nitrogen gas plasma. However, these methods fail to form thin high-quality oxynitride films with good controllability and reproducibility.
Conventionally practiced thermal oxynitridation using N.sub.2 O gas has involved the following problems: heating at high temperatures is required; the amount of nitrogen atoms incorporated into a formed oxynitride film is relatively small; and the quality of a silicon dioxide film is not sufficiently improved. According to conventionally practiced thermal oxynitridation using NO gas, a heating temperature is as low as about 900.degree. C., and the amount of nitrogen atoms incorporated into a formed oxynitride film increases somewhat; however, the method has involved the problem that the thickness of a formed oxynitride film cannot be made greater than a certain level. Conventionally practiced thermal oxynitridation using ammonia gas has involved the following problem. A formed oxynitride film contains a large amount of hydrogen, which serves as an electron trap, causing an impairment in film quality. Thus, in order to eliminate hydrogen, after an oxynitride film is formed, the film must be heated to a temperature of about 1000.degree. C. or must be oxidized.
Also, conventionally practiced direct oxynitridation using plasma has involved the problem that film quality is impaired due to plasma damage. Particularly, the generation of interface state not only impairs hot-carrier properties of a transistor but also causes an unstable threshold voltage of a transistor and impaired mobility of carriers, which induce a fatal problem for, particularly, fine-patterned devices.
Further, the fine patterning of an element requires a reduction in thermal treatment temperature. Accordingly, high-temperature heating has raised the problems of dopant diffusion and defect generation. In RF-plasma-activated oxynitridation of a silicon dioxide film, the use of NH.sub.3 plasma enables a relatively large amount of nitrogen atoms to be incorporated into the film, but causes a relatively large amount of hydrogen atoms to also be incorporated into the film. As a result, impairment in film quality is involved.
Also, the use of N.sub.2 plasma has involved insufficient improvement of film quality, since the amount of nitrogen atoms incorporated into a film is relatively small. (Refer to, for example, P. Fazan, M. Dutoit and M. Ilegems, "Applied Surface Science" Vol. 30, p. 224, 1987.)